This invention relates to a device and a method for polishing the surface of a tape-like metallic base material to a specified level of roughness, and more particularly to a polishing system and a polishing method for a tape-like metal serving as a base material for forming a functional thin film with the characteristic of superconductivity, ferroelectricity or ferromagnetism.
The surface processing of a material for a base plate is an important problem for products with a functional film formed and used above a tape-like metallic base material. A tape-like metallic base material is fabricated in the form of a tape generally by the process of cold rolling or hot rolling. By such a fabrication process, however, desired characteristics of a functional thin film cannot be obtained because of the scratches and crystalline defects caused by the rolling unless they are removed. For this reason, processes of not only removing scratches or crystalline defects but also making the surface flat and smooth have been practiced. Japanese Patent Publications Tokkai 8-294853 and 2001-269851 which are herein incorporated by reference, for example, have disclosed a device for and a method of polishing while pressing a traveling metallic belt of stainless steel onto a rotationally driven endless polishing belt. By either of these processes, however, the finally obtainable surface roughness is of the order of microns, which is not sufficient for forming a functional thin film thereupon. Depending upon the kind of the functional thin film to be formed, its characteristics are affected significantly by the crystalline characteristics of the surface of the tape-like metallic base material and the orientation characteristics of the crystal.
On the other hand, technologies for forming various types of orientation films on a polycrystalline base material are being utilized. In the fields of optical thin films, photomagnetic disks, wiring substrates, high-frequency transmission waveguides or filters and cavity resonators, for example, it is becoming a problem to form on a substrate a polycrystalline thin film having a good orientation characteristic with stable film quality. It is even more desirable to be able to form an optical thin film, a magnetic thin film or a wiring thin film with a good crystalline orientation characteristic directly on a base material since if the crystalline characteristic of the polycrystalline thin film is good, the film quality of the optical thin film, the magnetic thin film or the wiring thin film formed thereon is improved.
In recent years, superconducting oxides are coming to be attracting attention as superior superconductors with critical temperatures exceeding the temperature of liquid nitrogen but there are problems in order to put superconducting oxides of this kind into a practical use. One of these problems is the low critical current densities of superconducting oxides, and one of the big reasons for this is that the crystals of these superconducting oxides themselves have electrical anisotropy. It is known in particular that it is easy for an electric current to flow inside a superconducting oxide in the directions of a-axis and b-axis but it is difficult in the direction of the c-axis. In order to form a superconducting oxide on a base material and to use it as a superconducting body, therefore, it is necessary to form a superconducting oxide with good crystalline orientation characteristics on the base material, to orient the a-axis or the b-axis of the crystals of the superconducting oxide in the direction in which an electric current is to be passed, and to orient the c-axis of the superconducting oxide in another direction. U.S. Pat. No. 6,908,362, which is herein incorporated by reference, discloses such a method by forming a film of superconducting oxide after the surface of a tape of nickel or a nickel alloy is finely polished. Japanese Patent Publications Tokkai 6-145977 and 2003-36742, which are also herein incorporated by reference, disclose another method of providing an intermediate layer with controlled crystalline orientation on the surface of an elongated tape-like metallic base material and forming thereon a thin film of a superconducting oxide. The bonding characteristic among the crystalline particles is improved by this method and a high critical current density can be obtained.
All these prior art technologies indicate that it is important to polish the surface of the base material so as to make it flat and smooth. In order to accomplish even high critical current densities, however, it is necessary to form the surface of the tape-like metallic base material such that the surface is not only sufficiently flat but also easy to orient the crystals. It is therefore necessary that the surface of the tape-like metallic base material for forming the thin film be polished and finished uniformly on the order of nanometers and that a surface with good crystalline orientations be formed. It is also necessary to prevent oxide films or unwanted foreign objects from becoming attached to the finished surface. Since base materials to be used as a superconducting coil are processed in units of several hundred meters, furthermore, it is further necessary to polish the surface of such base material continuously at a high speed and uniformly to a surface roughness on the order of nanometers.